Fix request headers and use rustls for all targets

[andrius-puksta-sensmetry]

Include appropriate headers for all requests:

• User-Agent: sysand/<version>
• Accept: appropriate values for each type of request (kpar, json, text)

macOS native TLS (as used by Rust's security-framework) does not support TLS 1.3. Windows 10 secure-channel also does not support TLS 1.3 (Windows 11 does, but bindings don't expose it).
So use rustls for all targets. Certificate verification is done natively by each platform, so OS trust stores and rules are used.

Tests to do:

• index access
• GitHub:
  • public repo:
    • no authentication:
      • raw file in repo
      • raw file in a release
    • bearer token authentication:
      • file in repo via API
      • file in a release via API
    • user/pass authentication (user must be __token__, pass is PAT):
      • raw file in repo
      • raw file in a release
  • private repo:
    • bearer token authentication:
      • file in repo via API
      • file in a release via API
    • user/pass authentication (user must be __token__, pass is PAT):
      • raw file in repo
      • raw file in a release

[consideRatio]

I think this is sound, manual testing has failed for index due to the unrelated issue being fixed in another PR, but I have tried a few other things and considered how things could fail to provide the headers without seeing red-flags.

Let's go!